As integrated circuit geometries continue to shrink, manufacturers have increasingly turned to optical techniques to perform non-destructive inspection and analysis of the integrated circuits. The basis for these techniques is the idea that a substrate can be examined by analyzing the reflected energy that results when an optical beam is directed at the substrate. This type of inspection and analysis is known as optical metrology and is performed using a range of different optical techniques.
Metrology systems that use external means to induce thermal or plasma waves in a substrate under study are generally referred to as modulated optical reflectance systems. Modulated optical reflectance metrology systems are used to study a range of attributes, including material composition and layer thickness. Modulated optical reflectance systems and their associated uses are described in U.S. Pat. Nos. 4,634,290, 4,646,088, 4,679,946, 4,854,710, 5,854,719, 5,978,074, 5,074,699 and 6,452,685, the entire disclosures of which are incorporated herein by reference.
Another important use of modulated optical reflectance systems is measurement and analysis of the dopants added to integrated circuit substrates before and after their activation. Dopants are ions that are implanted into substrates, such as semiconducting substrates, such as during ion implantation. The duration of the ion implantation process is one of the factors that control the resulting dopant concentration. Generally, the ion energy used during the implantation process controls the depth of implant. Both ion concentration and depth are critical factors that determine the characteristics of the ion implantation process.
The ion implantation process damages the crystal lattice of the substrate as it receives the implanted ions. This damage is typically proportional to the concentration and depth of ions within the crystal lattice. This makes measurement of the damage an effective surrogate for direct measurement of dopant concentration and depth. Modulated optical reflectance systems are typically used to measure substrates at the completion of the ion implantation process to determine the extent of the damage. The modulated optical reflectance signal, which is proportional to the extent of the damage in the substrate, is then correlated to the implantation dose and other parameters of interest.
Dopant activation after the ion implantation step is typically performed by rapidly heating and cooling the substrate in a special chamber, or by scanning a localized heat spot from a laser beam across the surface of the substrate. This process is also known as annealing. During the annealing process, dopant ions may diffuse away from the positions that they had in the lattice after ion implantation, and form a concentration profile within the substrate (diffusion anneal). Alternately, the anneal dopant ions might stay within the same area where they were located after implant (diffusion-less anneal). The transition between the implanted region containing activated dopants and the non-implanted substrate is commonly referred to as a junction.
For advanced semiconductor manufacturing, it is generally desirable for the implanted and activated region to be shallow, typically no more than about five hundred angstroms, which depth is defined herein as an ultra-shallow junction. This is usually achieved by using a fast, diffusion-less anneal, such as a laser or spike anneal with dwell times (time at temperature) of less than a few milliseconds. While creating good carrier activation, defined as the ratio of activated ions to the total concentration of ions in the ultra-shallow junction, this type of annealing leaves certain defects at the bottom of the annealed junction. These defects are generally referred to as end-of-range damage. Depending on the implantation concentration and depth, end-of-range damage may negatively affect ultra-shallow junction quality and create junction leakage, including increased probability that the carriers will move out of the junction and into other portions the substrate. To adequately evaluate junction quality, it is important to characterize both carrier activation level and the end-of-range damage concentration in the ultra-shallow junction.
A number of techniques have been developed to characterize the effectiveness of the ultra-shallow junction process. Destructive and contact methods include secondary ion mass spectroscopy, transmission electron microscopy, spreading resistance depth profiling, and sheet resistance (Rs or 4-point probe) technologies. Transmission electron microscopy has been the only technique used to evaluate end-of-range defect density. Although such techniques are capable of providing ultra-shallow junction profile information, it is at the expense of turn-around times, usually measured in days or even weeks, or at the expense of damaging the surface with contacts or breaking the substrate.
What is needed, therefore, is a reliable, non-contact, and non-destructive technique to monitor both carrier activation and end-of-range defect concentration in ultra-shallow junctions.